1. Field of the Invention
The present invention is directed to vertical resonator laser diodes (vertical cavity surface emitting laser diode).
2. Discussion of the Related Art
Vertical resonator laser diodes (VCSELs) have a number of advantages over traditional laser diodes with longitudinal resonator. These include, low beam divergence which is advantageous for a number of applications, such as fast optocouplers, sensors, encoders. In the standard embodiments of a VCSEL, the vertical resonator is often produced in a mesa structure, whereby a metallic contact for the current injection is located on the surface of the mesa. In order to electrically connect the VCSEL chip with a housing, a metallic surface is additionally applied on the component as a terminal contact surface (bond pad), this being electrically connected to the mesa contact. At least three mask levels are required for manufacturing this conductive connection between the contacts, particularly in the manufacture of 850 nm VCSELs on GaAs substrates that emit toward the overgrowth side of the substrate. This represents a considerable manufacturing outlay and, thus, a critical cost factor.
In the inventive VCSEL, a vertical resonator structure in a mesa having a contact applied on the top side. A contact surface (bond pad) provided for external electrical connection, particularly for the electrical connection to a housing, is located on the same top side in the proximity of the contact and is preferably fashioned lying completely on one side of the contact. The bond pad, just like the contact, can be directly electrically conductively connected to the semiconductor layer to be contacted, in that a low-impedance metal-semiconductor contact is also formed between the bond pad and this semiconductor layer. In an especially preferred development, the contact and the bond pad are parts of the same electrically conductive contacting layer, preferably a metallization layer. Thus, these parts immediately adjoin one another and can be identified as contact and or bond pad by their position with reference to the planned radiation emission and by their geometrical dimensions.
The current bundling in the active layer needed for an efficient laser operation can be achieved with an aperture that electrically insulates a region lying outside the predetermined current path. Such an aperture can be formed by a lateral oxidation of one or more oxidizable semiconductor layers in the layer structure. For example, a semiconductor material suitable for this purpose is AlGaAs. An aperture layer composed of such a material can be oxidized down into a predetermined depth proceeding from the sidewall of a mesa structure formed by the layer structure, namely to such an extent into the mesa, that the projection of the bond pad perpendicular to the layer planes lies in the insulating region of the layer. The current path is thus constricted onto the region of the perpendicular projection of the contact. Consequently, the bond pad serves only for the external electrical connection without influencing the current injection, which allows an arrangement of the bond pad that is laterally adjacent to the contact but asymmetrical relative to the center of the radiation emission. The preferred embodiment of the inventive VCSEL therefore differs from traditional VCSELs in terms of the lateral dimension of the insulating region round about the aperture. This insulating region has a larger dimension in a radial direction of the mesa, i.e. in the layer plane from the center to the sidewall of the mesa, than that of a typical dimension of the bond pad of, for example, 90 xcexcm and at least approximately 50 xcexcm.
It is an object of the present invention to provide a VCSEL and a method of making it, having a top-side contact and a bond pad for external electrical connection of the contact, that is significantly easier to manufacture compared to traditional embodiments.
It is another object of the invention to provide a VCSEL and a method of making it, having a larger dimension in the layer plane from the center of the sidewall of the mesa.
It is a further object of the invention to provide a VCSEL and a method of making it, having a bond pad that serves only for the external electrical connection without influencing the current junction.